An antenna tower mounted preamplifier offers up to a ten DB improvement in receiver sensitivity over preamplifier systems located at the base of the tower in that the former preserves the signal-to-noise ratio existing at the base of the antenna by amplifying signals before same are reduced in level by tower to ground transmission line losses. In that a tower mounted preamplifier is not readily available for maintenance in case of system failure, it has been the practice to provide a system which permits the tower mounted preamplifier to be bypassed and signals carried by the transmission line to a backup or secondary preamplifier located at ground level. However, the improvement in system sensitivity achieved by using a tower mounted preamplifier system is lost when the system is switched to the backup preamplifier, which is not located in the tower.
The use of an alternate amplifier circuit to be used in the event of a primary circuit failure is well known but all of the existing systems fail to provide a backup system suitable for a tower mounted application with remote control. For instance, J. Maggio et al, U.S. Pat. No. 2,229,108 on "Switching Of Spare Repeater Sections" issued Jan. 21, 1941 discloses the broad concept of a spare amplifier circuit in combination with a plurality of amplifier circuits. The system requires an intricate array of relays to provide a single spare circuit for a three circuit system and the extensive hardware required to provide the switching increases the probability of failure to a point which far exceeds any benefits which may be derived in overall reliability from inclusion of the spare circuit. An even more serious shortcoming of the Maggio et al system is the fact that the multi-pole, multi-throw relays incorporated in the system are satisfactory for high frequency operation but completely unsatisfactory for use with preamplifiers where RF frequencies are encountered.
Other examples of backup amplifier circuits may be found in Haddock, U.S. Pat. No. 1,396,745; Blattner, U.S. Pat. No. 1,472,455; Toomy, U.S. Pat. No. 1,525,054; Daly, U.S. Pat. No. 2,647,176; and Karlson, U.S. Pat. No. 2,773,944. None of the techniques utilized by these references lends itself to the use in a remote tower or to preamplifier systems operating in the RF range.